PROJECT ABSTRACT The overall goal of this project is to understand quorum sensing: the process of cell-cell communication in bacteria. The proposed research will probe the exquisite capacity of quorum-sensing receptors to differentiate between closely related, extracellular signal molecules and to exploit their structural differences to regulate signaling activity (Aim 1) The research will define the mechanisms by which the information encoded in the extracellular chemicals is relayed internally and interpreted by the vibrio Qrr small regulatory RNAs to precisely control the global gene expression program underpinning collective behaviors (Aim 2). Aim 3 focuses on our recent discovery of a new quorum-sensing system consisting of a novel extracellular signal molecule, a new cytoplasmic receptor, a new regulatory small RNA, and its 17 target genes. The new extracellular quorum- sensing signal molecule is likely generated by the host microbiota and this is relevant during Vibrio cholerae infection. The proposed research is multidisciplinary, employing microbiology, genetics, biochemistry, structural biology, chemistry, physics theory, evolution, imaging, and engineering. At the most general level, the work will provide insight into intra- and inter-species communication, population-level cooperation, and the network principles underlying signal transduction and information processing. At a more specific level, the research will advance understanding of how quorum-sensing receptors accurately detect, distinguish between, and decode information contained in extracellular small molecules and how that information, once transduced into the cell, is conveyed to control gene expression, and ultimately behavior. At a practical level, my group's investigations could lead to strategies for controlling quorum sensing, potentially resulting in th development of anti- microbial drugs aimed at bacteria that use quorum sensing to control virulence and biofilm formation, and improved industrial production of high-value natural products. As in the previous project period, two vibrio species (Vibrio cholerae and Vibrio harveyi) are under study, providing insight into how closely related species have, through evolution, uniquely optimized their quorum-sensing circuits to perform distinct biology.